Two things: First, they very likely don't make most of their money off of their loot box sales from the few games they own that have them. (CS, Dota 2, TF2).

Secondly, you're just full of shit and making up that "fact". As a private company, none of their stuff is completely public record, but using various trackers and estimates and sales numbers from game studios, Steam sells north of 700,000,000 games a year and makes a $billion or so from cs and tf2 stuff.

Since the average game sale amount is around $15; $15 X 700,000,000 = 10,500,000,000 ÷ valves cut of 30% = $3,150,000,000

Now not all studios pay that 30%. Some have lower deals based on volume and notoriety, but it's still safe to assume that their game sales make over two billion a year. AKA well over their loot box money.